linux-kernel - Re: [PATCH v4 10/29] x86/die: Don't try to recover from an OOPS on a non-default stack

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Message-ID: <20160702183451.e2nlfkfxlsuicbqd@treble>
Date:	Sat, 2 Jul 2016 13:34:51 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Andy Lutomirski <luto@...nel.org>, x86@...nel.org,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	Nadav Amit <nadav.amit@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Brian Gerst <brgerst@...il.com>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jann Horn <jann@...jh.net>,
	Heiko Carstens <heiko.carstens@...ibm.com>
Subject: Re: [PATCH v4 10/29] x86/die: Don't try to recover from an OOPS on a
 non-default stack

On Sat, Jul 02, 2016 at 07:24:41PM +0200, Borislav Petkov wrote:
> On Sun, Jun 26, 2016 at 02:55:32PM -0700, Andy Lutomirski wrote:
> > It's not going to work, because the scheduler will explode if we try
> > to schedule when running on an IST stack or similar.
> > 
> > This will matter when we let kernel stack overflows (which are #DF)
> > call die().
> > 
> > Signed-off-by: Andy Lutomirski <luto@...nel.org>
> > ---
> >  arch/x86/kernel/dumpstack.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
> > index ef8017ca5ba9..352f022cfd5b 100644
> > --- a/arch/x86/kernel/dumpstack.c
> > +++ b/arch/x86/kernel/dumpstack.c
> > @@ -245,6 +245,9 @@ void oops_end(unsigned long flags, struct pt_regs *regs, int signr)
> >  		return;
> >  	if (in_interrupt())
> >  		panic("Fatal exception in interrupt");
> > +	if (((current_stack_pointer() ^ (current_top_of_stack() - 1))
> > +	     & ~(THREAD_SIZE - 1)) != 0)
> 
> Ugh, that's hard to parse. You could remove the "!= 0" at least to
> shorten it a bit and have one less braces level.
> 
> Or maybe even do something like that to make it a bit more readable:
> 
>         if ((current_stack_pointer() ^ (current_top_of_stack() - 1))
>                         &
>              ~(THREAD_SIZE - 1))
>                 panic("Fatal exception on non-default stack");
> 
> Meh.

A helper function would be even better.

The existing 'object_is_on_stack()' can probably be used:

	if (!object_is_on_stack(current_top_of_stack()))
		panic("...");

Though that function isn't quite accurately named.  It should really
have 'task_stack' in its name, like 'object_is_on_task_stack()'.  Or
even better, something more concise like 'on_task_stack()'.

-- 
Josh